Clark Ashton Smith
Clark Ashton Smith was an American writer of weird horror, pulp fantasy and science fiction novels, poems and serials. He is one of "The Big Three of Weird Tales", the three men who published most prolifically in that most well-known and long-lasting of Weird Fantasy dedicated magazines, alongside the more famous H.P. Lovecraft and Robert E. Howard, author of the iconic Conan the Barbarian tales. Although he isn't as well-known as them, he had a significant impact; Clark was a co-creator for the world of Hyperborea, the primeval "lost past" world in which Robert's own Conan and King Kull Sword & Sorcery works were set, and corresponded frequently with Lovecraft, resulting in many of his weird races and deities being promoted in the latter's works. It is also worth noting that Lovecraft praised Clark Ashton Smith as one of the modern (early 20th century) masters of the weird tale.
Klarkash-Ton's Hyperborean tales are more obscure than Howard's, and overlap in theme if not in setting with his Poseidonis novels, set in a remnant of the sunken continent of Atlantis that avoided sinking into the ocean; his stories for both of those settings revolve around a magical culture characterized by bizarreness, cruelty, death and postmortem horrors. Averoigne is Smith's version of pre-modern France, comparable to James Branch Cabell's Poictesme, taking place in an alternate Earth where magic is very real, just illegal, and cracked down upon by the church. Zothique - debuting late in his career - exists millions of years in the future. It is "the last continent of earth, when the sun is dim and tarnished". This inspired Jack Vance's Dying Earth series and then Gene Wolfe's New Sun.
Smith is most of interest to /tg/ for his role in inspiring Gary Gygax to write Dungeons & Dragons, as he has long been listed in the inspirations for D&D as a whole. The adventure module X2: Castle Amber invokes a cross-over between Mystara and Averoigne.
This was all super-annoying to 1980s kiddies who could never track down his much-hyped work at the bookstores or libraries. But then the Internet came along and Eldritch Dark posted whatever they legally could. Those mad lads at Nightshade Press recently Got Shit Done and edited every story the man ever wrote from the original manuscripts - definitely an improvement over ED's 1990s-flava webpages, much as they were appreciated then. Seriously, go out and buy all five Nightshade volumes, you will not regret it.
Perhaps because Eldritch Dark made Clark's Zothique writings (especially) more-or-less readily available, its necromancers are cited as a direct inspiration in The Complete Necromancer's Handbook; Wolfgang Baur also quotes such content. And in Pathfinder Zothique's ghoul-worshipped Great Old One Mordiggian became a deity.